I had the same problem on i386 just a couple weeks ago. I ended up wiping
the whole machine (had some partition problems, and I was trying to 3-way
bloze, Debian, and BeOS), but I suspected the problem might have been
pam-related.

this is probably not the problem, but current OpenSSH still has a bug
where it looks for /etc/pam.d/sshd that does not exist
(/etc/pam.d/ssh exists, and is what it should look for)

since this has happened before and was fixed and now is broken again
I just hard linked ssh sshd in /etc/pam.d that way I don't have to
worry about this dumb bug. (i have /etc/pam.d/other set to default
deny as well so screwed up pam files are not tolerable for me)